In pattern matching to determine whether or not two patterns are identical to each other, the pattern to be matched may include both of a feature changing with time and a feature not changing with time. In such a case of matching the patterns including both of the feature changing with time and the feature not changing with time, there is a problem that if two patterns are obtained at a considerably long interval, the feature changing with time changes, which makes it difficult to perform the matching.
For example, in a case of matching finger surface patterns, a finger surface pattern to be matched includes both of a feature as fingerprint ridges, which does not change with time, and a feature of wrinkles, scars, or roughness on a finger surface, which changes with time. For personal identification, a finger surface pattern at some time point is registered as a template, at a later date, for reference, a finger surface pattern taken at the time may be compared with the template. However, if a time interval between the registration and the reference is long, since the feature of wrinkles, scars, or roughness has changed, there is a risk that the finger surface pattern at the reference may be judged to be a pattern different from the finger surface pattern of the template, even though the both are taken from a finger of a same person. On the other hand, if an allowable error is set large such that the features changing with time are judged to be identical, a risk rises that a finger surface pattern of another person is judged to be a pattern identical to the template.
As a method for dealing with such a problem, there is a method that uses only features not changing with time for evaluation. For example, a fingerprint/palmprint image processing device described in Japanese patent publication (JP-A-Heisei 9-167230) is configured to extract only ridge features that do not change with time. As described, by using only features not changing with time for matching, it is not necessary to worry about time change of the feature, and therefore patterns taken at a long interval can be matched. However, it is difficult to match patterns including a feature changing with time with a high accuracy. That is, as described in Japanese patent publication (JP-A-Heisei 9-167230), if features used for matching are limited to features not changing with time in order to eliminate an influence of features changing with time, amount of information usable for the matching is reduced. There is also a case that a combination of the features changing with time and the features not changing with time provides sufficient amount of information for matching, but only the features not changing with time provide insufficient amount of information. In this case, by matching only the features not changing with time, high accuracy matching cannot be performed. For example, in a case that a method described in Japanese patent publication (JP-A-Heisei 9-167230) is used and that features to be used for matching are limited to ridge features, if a region from which ridges can be extracted is small due to an influence of wrinkles, scars or the like, ridges sufficient for the matching cannot be obtained, and matching accuracy is reduced.
As another method for dealing with time change of a pattern, there is a method in which a template registered at some time point is not used after long time has elapsed, but updated at constant interval or for each matching. For example, according to a personal authentication apparatus described in Japanese patent publication (JP-P2006-092491A), by using a matching result for data obtained from image data which is took at the last authentication, an authentication is possible even if a pattern has changed with time. However, in a method described in Japanese patent publication
(JP-P2006-092491A), it is difficult to safely perform matching. That is, in a method in which a template to be compared is update as described in Japanese patent publication (JP-P2006-092491A) in order to eliminate an influence of pattern change due to the change with time, there is a risk that a different pattern may be registered as the template. Once the different pattern is erroneously authenticated as an identical pattern, the erroneously authenticated pattern is registered as the template at the update of template. For example, in the method described in Japanese patent publication (JP-P2006-092491A), since matching is carried out by using an image having succeeded in authentication at the last time, a single erroneous authentication causes that the erroneously authenticated pattern is used as a correct registration pattern next time or later. Thus, there is a risk that a template is erroneously updated, and it is difficult to safely perform matching.
According to a personal identification system based on a fingerprint described in Japanese patent publication (JP-P2004-110839A), a fingerprint which is registered for the first time and a fingerprint which is matched for the last time are saved. Then, by performing matching by using each of them, matching is possible even if a pattern has changed with time. A method described in Japanese patent publication (JP-P2004-110839A) attempts to achieve both of robustness to pattern change and safeness of matching by performing comparison by using each of the fingerprint registered for the first time and the fingerprint matched for the last time. However, if a pattern has changed, the pattern cannot be directly matched with the fingerprint registered for the first time. For this reason, in a case that the pattern is not judged to be identical if the matching with the fingerprint registered for the first time cannot be performed, the robustness to the pattern change is not achieved. If only a result of matching with the fingerprint matched for the last time is used in order to achieve the robustness to the pattern change, similarly to Japanese patent publication (JP-P2006-092491A), there is a risk that an erroneous pattern may be registered. Even when the method described in Japanese patent publication (JP-P2004-110839A) is used, it is still difficult to achieve matching of a pattern including time change with both of high accuracy and safety.
Japanese patent publication (JP-A-Heisei 11-025268) describes an invention of a fingerprint matching device that separately stores features in a feature information main storage unit and a feature information sub storage unit. The fingerprint matching device performs updating in order to store better features and improve a matching performance. The reason why the features are stored in the two storage unit is to update the information stored in the main storage unit when a specific condition is satisfied.